Due to constantly increasing demand for high data rates, cellular networks that can meet this demand are required. One major issue for cellular network operators is finding ways to evolve their existing cellular networks to provide higher data rates. In this respect, the following approaches have been proposed to provide higher data rates in existing cellular networks: (i) increasing the density of existing macro base stations, (ii) increasing cooperation of existing macro base stations, or (iii) deploying pico base stations in areas where high data rates are needed within a macro base station grid. Approaches (i) and (ii) are problematic in that it is often difficult to find new locations for macro base stations especially in urban environments and both result in significant costs and delays. In addition, increasing the density of macro base stations would lead to a significant increase in signaling due to frequent handovers for users moving at high speeds.
A cellular network including the pico base stations in approach (iii) is referred to herein as a “heterogeneous cellular network” or a “heterogeneous deployment.” The pico base stations may also be referred to as micro base stations or low power nodes (LPNs). Pico base stations are advantageous because it is easier and more cost efficient to find sites for pico base stations. In addition, pico base stations are expected to be more cost-efficient than macro base stations and their deployment time is expected to be shorter. With heterogeneous cellular networks, the macro base stations (i.e., the macro layer grid) can serve mainly users moving at high speeds or wider areas where demand for high data rates is relatively low. The pico base stations (i.e., the pico layer grid) can then cater to areas with many users desiring high data rates but having lower mobility (i.e., “hotspots”).
FIG. 1 illustrates a conventional heterogeneous cellular network 10. As illustrated, the heterogeneous cellular network 10 includes macro base stations 12-1 through 12-4 (generally referred to herein collectively as macro base stations 12 and individually as macro base station 12) having corresponding coverage areas, which are referred to herein as macro base station cells 14-1 through 14-4 (generally referred to herein collectively as macro base station cells 14 and individually as macro base station cell 14). The heterogeneous cellular network 10 also includes pico base stations 16-1 through 16-4 (generally referred to herein collectively as pico base stations 16 and individually as pico base station 16) having corresponding coverage areas, which are referred to herein as pico base station cells 18-1 through 18-4 (generally referred to herein collectively as pico base station cells 18 and individually as pico base station cell 18). The transmission power of the pico base stations 16 is much smaller than the transmission power of the macro base stations 12, which results in the pico base station cells 18 being much smaller than the macro base station cells 14.
In this example, each of the pico base station cells 18 is within a corresponding one of the macro base station cells 14. However, the heterogeneous cellular network 10 is not limited thereto. Some of the macro base station cells 14 may include no pico base station cells 18 while other macro base station cells 14 may include one or more pico base station cells 18. Further, it should be noted that the pico base station 16-1 is referred to herein as “neighboring” the macro base station 12-1. Likewise, the pico base stations 16-2, 16-3, and 16-4 are neighboring pico base stations for the macro base stations 12-2, 12-3, and 12-4, respectively. Thus, a pico base station (e.g., one of the pico base stations 16) whose pico base station cell borders the macro base station cell of a macro base station (e.g., one of the macro base stations 12) is referred to herein as a neighboring pico base station of the macro base station.
In the conventional heterogeneous cellular network 10, cell selection is performed such that user equipments (UEs) (e.g., mobile devices) are connected to the macro base station 12 or pico base station 16 providing the best downlinks for those UEs. In other words, a UE measures a received signal strength for a downlink from the nearest macro base station 12 and a received signal strength for a downlink from the nearest pico base station 16. If the macro base station 12 provides the best downlink (i.e., has the highest received signal strength at the UE) for the UE, then the macro base station cell 14 is selected as the serving cell for the UE. Otherwise, if the pico base station 16 provides the best downlink for the UE, then the pico base station cell 18 is selected as the serving cell for the UE. For Long Term Evolution (LTE) networks, the received signal strength is typically measured by Reference Signal Received Power (RSRP) measurements.
One issue that arises when using the conventional cell selection scheme is that the UEs are connected to the macro or pico base stations 12 or 16 that provide the best downlink for the UEs but not necessarily the best uplink for the UEs. This issue is illustrated in FIG. 2. More specifically, FIG. 2 illustrates a first cell edge, or border, 20 between the macro base station cell 14 and the pico base station cell 18 determined based on downlink received signal strength according to the conventional cell selection scheme. FIG. 2 also illustrates a second cell edge, or border, 22 between the macro base station cell 14 and the pico base station cell 18 determined based on uplink path loss. Thus, as shown in FIG. 2, the conventional cell selection scheme that performs cell selection to provide the best downlink does not necessarily provide the best uplink. Specifically, using the conventional cell selection scheme that provides the best downlink, a UE 24 would be connected to the macro base station 12. As a result, the UE 24 is provided with the best downlink, but does not have the best uplink. As a result, using the conventional cell selection scheme, data rates, particularly in the uplink direction, are not optimal. Specifically, the UE 24 is not connected to the pico base station 16 providing the best uplink for the UE 24. Also, the links for other UEs connected to the pico base station 16 are negatively affected by higher uplink interference generated by the UE 24.
As such, there is a need for systems and methods for controlling cell selection in a heterogeneous cellular network to provide increased data rates.